The present invention relates to a method and apparatus for measuring the size distribution of aerosols over a wide particle size range. Specifically, the invention relates to the measurement of particles suspended in a gas, which is referred to as an aerosol. The most common carrier gas is air, but other gases, such as nitrogen, helium, argon, CO2, and other gases, may also be the media for particle suspension. The particles can be solid, liquid, or a mixture of both.
In the ambient atmosphere, particles may exist over a size range from about 2 nanometers (nm) to over 50,000 nm in diameter, with particles in the 10 nm to 10,000 nm range being the most important from a health and safety standpoint. No single device currently exists that can measure particles over this range. The wide-range particle counter (WPC) described herein makes this possible.
Particle counters now available have a limited operable range of sizes, and several different particle counters are needed to properly analyze aerosols.
Aerosols occur both in nature and in the human environment. They are important in scientific research and in technical applications. Aerosol particles in the atmosphere can scatter light and affect atmospheric visibility. When inhaled, the suspended particles can deposit in the lungs to cause potential health effects in humans. Aerosol particles often need to be measured so the sources of the particles can be controlled or precautions taken if the sources cannot be controlled.
Aerosols are also generated on purpose for scientific and technical applications. In laboratory studies, for instance, aerosols with controlled size distribution are needed to test filters and other particle collectors to determine their efficiency. In medical applications, drug compounds are frequently generated in aerosol form for delivery to the lungs for disease treatment. The particle size distribution is important because particle size determines the specific regions of the lungs where the inhaled particles will deposit, hence the effectiveness and efficacy of the inhaled drugs. In all cases in this specification, a gas containing suspended particles shall be referred to as an aerosol, with no limitation being made as to the chemical nature of the particles and that of the gas, and their respective physical states.
One of the most widely used aerosol-measuring instruments presently is the optical particle counter (OPC) first described in U.S. Pat. No. 2,732,753 (O""Konski). In an OPC, the aerosol is passed through a beam of light to cause optical scattering. The scattered light signal from each particle is then detected and related to particle size. The OPC is capable of detecting particles to a lower size limit of about 100 nm in diameter, with some special OPCs having been designed to detect particles as small as 60 nm in diameter or a characteristic dimension.
Another particle-measuring instrument is the condensation nucleus counter (CNC), also referred to as a condensation particle counter. The most widely used CNC is that based on U.S. Pat. No. 4,790,650 (Keady). In this CNC, the aerosol is first saturated with the vapor of a working fluid at an elevated temperature. A typical working fluid is butyl alcohol, and a typical saturator temperature is 35xc2x0 C. The vapor-laden aerosol then passes through a condenser, typically kept at 5xc2x0 C. to cool the gas and cause the vapor to condense on particles to form droplets. The droplets are then counted by optical scattering, as in a conventional OPC. The CNC is capable of detecting particles below the lower size limit of the OPC, since droplets formed by vapor condensation are considerably larger than the particles themselves, thus making them easier to detector by light scattering.
Since a CNC is only capable of counting particles, but not measuring the particle size, a CNC must be combined with a size-analyzing device, such as a mobility analyzer, in order to both determine the size and the particles count. A differential mobility analyzer (DMA) is usually used for size determination. The DMA method of size classification is based on the electrical mobility of singly charged particles, i.e. particles carrying a single electron unit of charge. Liu and Pui (1974) and Knutson and Whitby (1975) were the developers of the DMA for this application. The publications explaining this DMA method are: xe2x80x9cA Submicron Aerosol Standard and the Primary, Absolute Calibration of the Condensation Nuclei Counter,xe2x80x9d Benjamin Y. H. Liu, David Y. H. Pui, Journal of Colloid and Interface Science, vol. 47, No. 1, Apr. 1974; and xe2x80x9cAerosol Classification by Electric Mobility: Apparatus, Theory, and Applications,xe2x80x9d Journal of Aerosol Science, 1975 pp. 443-451, W. O. Knutson and K. T. Whitby.
Recent improvements in the DMA are described in the article xe2x80x9cDesign and Testing of an Aerosol/Sheath Inlet for High Resolution Measurements with a DMA,xe2x80x9d Da-Ren Chen, David Y. H. Pui, George W. Mulholland, and Marco Fernandez, Journal of Aerosol Science, Vol. 30, No. 8, pp. 983-999, 1999 by Chen et al (1995). The development of the nano-DMA for particle measurement below 50 nm in particle diameter is disclosed by Pui et al in U.S. Pat. No. 6,230,572 B1. These recent developments further improved the accuracy and range of the DMA devices.
The DMA method of size classification relies on the fact that the electrical mobility of a singly charged particle is inversely related to particle size. A polydisburse aerosol containing singly charged particles over a range of sizes can be classified according to size in an electric field and produce a nearly monodisburse aerosol within a narrow range of electrical mobilities and thus the produced aerosol contains particles of substantially the same size. The classified aerosol can then be counted by a CNC. The DMA is generally limited to particles smaller than about 500 nm in diameter.
All aerosol-measuring instruments have certain inherent size limits. In the case of the DMA, the limit is due to the low electrical mobility of large particles. As the particle size increases, the electrical voltage needed to classify the particle by electrical mobility also increases. At the usual flow rate used in differential mobility analysis, voltages as high as 10,000 Volts may be needed to classify particles at a diameter of 500 nm. For this reason, mobility analysis is seldom used beyond an upper size limit of about 500 nm.
On the other hand, the OPC is limited in the particle size it can satisfactorily detect due to the scattered light signal from a particle generally decreasing with decreasing particle size. Below about 100 nm, the scattered light signal begins to enter the so-called Rayleigh scattering regime, where the signal varies approximately as the sixth power of particle size. A factor-of-two reduction in particle size would thus lead to approximately a 64-fold reduction in the scattered light signal. Detecting small particles below 100 nm becomes increasingly more difficult, even when using a high-powered lasers as light sources, collecting optics with a high numerical aperture, and sensitive photo-detectors. Although optical particle counters have been designed to detect particles as small as 60 nm in diameter, the equipment needed is generally large and expensive. For this reason, high sensitivity optical particle counters are not widely used.
In principle, optical particle counters can be further improved to detect particles smaller than 60 nm. With further advance, even smaller particles may be detectable. However, advances in optical particle counting technology have not made the technology more useful for aerosol measurement over a wide size range. Designers of optical particle counters have not recognized the issues related to wide range particle counting and the special requirements that must be met in order to measure particles over a wide size range. A requirement that is illustrated with the following example.
In the ambient atmosphere, the aerosol size distribution generally follows Junge""s law, which states that the concentration of aerosol particles larger than a certain size is inversely proportional to the 3rd power of particle size. If the atmospheric particle concentration larger than 50 nm is, say 30,000 particles per cc, then the concentration of particles larger than 500 nm would be a factor of 1,000 lower, or on the order of 30 particles per cc. For particles larger than 5,000 nm, the concentration would be a million times lower, or on the order of 0.03 particles per cc.
The sharply declining concentration of large particles in the atmosphere indicates that even if a single detector is developed that can detect particles over a wide size range, say, from 50 nm to 10,000 nm in diameter, the detector, when operated at a specific sampling flow rate, would result in very high particle count rates in the small particle range, and a very low count rate in the large particle range.
For instance, at a sampling flow rate of 1 liter per minute (1 pm), i.e. 1,000 cc per minute, each minute would give rise to 30,000,000 particles in the 50 nm to 500 nm diameter range that need to be counted. Such a count rate is generally too high and would exceed the count rate limitation of the current optical counter technology. On the other hand, each minute of sampling by the detector would only yield 30 counts for particles in the greater than 5,000 nm range. Such a particle count is usually too low for statistically accurate purposes.
In order to count atmospheric fine particles in the 50 nm to 500 nm range at a more reasonable rate, the flow rate of the detector may be reduced to, say, 0.1 lpm so that only 3,000,000 particles need to be counted each minute. At such a sampling flow rate, the detector would yield only 3 particle counts in the greater than 5,000 nm range each minute, thus worsening the statistical accuracy of the large particle count. On the other hand, if the sampling flow rate is increased to, say, 10 pm so that 300 particles in the greater than 5,000 nm range can be counted each minute to improve the statistical counting accuracy for large particles, 300,000,000 particles would need to be counted in the 50 nm to 500 nm range, thus worsening the count-rate requirement of the counter for fine particles.
This example illustrates why the OPC is unable to measure aerosols over a wide size range, and why the conventional OPC by itself is inherently incapable of making such measurements with accuracy over the entire particle size range of interest in aerosols.
The present invention is a single measuring instrument built on a common chasis or a single platform, using multiple sensors provided with appropriate flow rates to detect and measure aerosol particles over a wide size range, typically from 10 nm to 10,000 nm in diameter, and greater for example, from 2 nm to 50,000 nm. Instrument sections measuring the 10 nm to 10,000 nm particle size are termed wide-range particle counters (WPC) and the 2 nm to 50,000 nm range instrument sections are termed ultra-wide range, particle counters (UWPC). These instruments make it possible to carry out measurements that are not possible with currently available instrumentation.
The WPC described in this specification is based on the novel combination of multiple sensors or detectors that combine optical detection with electrical mobility analysis to form a single device covering a wide particle size range. Each sensor is optimized in terms of particle measurement range, aerosol flow rate, reduced particle loss in sampling lines, and optical and electrical designs.
The measuring instrument of the present invention is simple in design and yet capable of performing the measurement automatically over a wide size range.
The lower limit is preferably 2 nm to 20 nm, and the upper limit can be anywhere between 5,000 nm and 50,000 nm.
The instrument includes controls for controlling operating parameters in order to insure reliable instrument operation, accuracy of measurement, and ease of use.
The number of sensors, flowmeters, pumps, and other components are minimized so that a rather complicated instrument like the WPC can be simplified and manufactured at a reasonable cost.
The resulting instrument described herein is estimated to weigh less than 35 lbs., making the device quite portable and convenient to use.